1. Field of the Invention
The present invention relates to a multi-stage sprocket assembly for a bicycle where a drive chain is shifted from a larger sprocket to a smaller sprocket.
2. Description of the Prior Art
According to a conventional multi-sprocket assembly, disclosed in e.g. a Japanese published patent gazette No. 56-3230, the large sprocket of the assembly includes a disengagement-facilitating means for facilitating disengagement of the drive chain from teeth of this large sprocket when the drive chain is shifted from the large sprocket to the small sprocket to change a driving speed of the bicycle. Specifically, this disengagement-facilitating means comprises reduction in the height of some of the teeth of the large sprocket relative to the other teeth in order to facilitate the chain disengagement at this portion of the large sprocket.
According to another conventional multi-stage sprocket assembly disclosed in a Japanese published utility model gazette No. 56-3265, the large sprocket has, in its face facing the small sprocket, a concavity to form a stepped portion for receiving the shifting drive chain when the chain is shifted from this large sprocket to the small sprocket disposed adjacent thereto. Specifically, this stepped portion is provided at a position higher than the dedendum of the teeth of the large sprocket so as to avoid locking of the shifting chain segment to the large sprocket.
In the course of chain shift from the large sprocket to the small sprocket, the chain may ride on a tooth crest of the small sprocket thus intefering with the chain shift without proper phase arrangement between the teeth of the large and small sprockets.
However, according to the former-mentioned convention, while the chain shift is initiated at the predetermined portion of the small sprocket, the large sprocket has no predetermined portion for reliably receiving this chain segment. Accordingly, there still tends to occur the problem of the chain riding over the tooth crest of the small sprocket, depending on the magnitude of tension acting on the shifting chain segment.
Further, for reliable and smooth chain shift, the sprocket assembly must be provided with proper teeth phase arrangement between the large and small sprocket. On the other hand, according to the latter-mentioned convention, its stepped portion is adapted merely for receiving a lower edge of the chain. Accordingly, even if the disengaging portion of the chain from the large sprocket is displaced only by one pitch amount from its proper phase position relative to the small sprocket, the displacement may cause the problem of the chain riding over the tooth crest of the small sprocket. Therefore, this type of convention suffers significant restriction in the freedom of phase relationship design.
Taking the above-described states of the art into consideration, the primary object of the present invention is to provide an improved multi-stage sprocket assembly for a bicycle which can assure smooth and reliable chain shift action from the large sprocket to the small sprocket.
The secondary object of the invention, in addition to the primary object, is to provide the assembly with greater freedom in designing of the phase relationship between the large and small sprocket for proper and smooth chain action from the large sprocket to the small sprocket.